Actually lot has changed with cell phones today, i'll let you know what i know

1. Cell phone network is very much based on OSI refrence model, so it
has its pros and cons.

2. Eavesdropping really depends upon the underlying technology like
GSM or CDMA. It is realively ro eavesdrop a GSM conversation and almost
impossible on CDMA.

3. At the backend like SMS or IVR systems, they are really computer systems and
in *computer networks* , there are Cisco H232 routers, Oracle databases, unix
systems and Microsoft SQL Servers.

4. To eavesdrop you need expensive hardware thats why it is not so common.

Any how check out htp://www.cellphonehacks.com